TokenMissingError fix

Closes #1051.

Distinguish between TokenMissingError raised during parsing from the one raised during evaluation in IEx.

Have I taken a correct direction here? I don't know how to get `file` from `config.scope` [here](https://github.com/elixir-lang/elixir/pull/1081/files#L0R20).

I should also add a test for this. We don't have any existing tests for IEx, do we?